This invention relates generally to multi-stage centrifugal pumps, and, more particularly, to pumps of this kind that generate high-axial thrusts.
Multi-stage centrifugal pumps ordinarily include a plurality of impellers mounted on a common impeller shaft rotatable within a housing. The impellers are arranged in series, with the discharge outlet of each impeller delivering pumped fluid or pumpage to the suction inlet of the next impeller. An inlet port in the housing delivers the fluid to the suction inlet of the first-stage impeller, and an outlet port in the housing receives high pressure fluid from the discharge outlet of the final-stage impeller.
Each impeller stage imparts an axial thrust to the rotating impeller shaft, and the combined axial thrusts of the impellers must be resisted or balanced in some manner. Frequently, the axial thrusts are balanced by arranging the impellers in a particular order and orientation on the shaft. For example, a first set of impeller stages can be arranged in side-by-side relationship at one end of the shaft, with all of those impellers oriented in a first direction, and a second set of impeller stages can be arranged in side-by-side relationship at the other end of the shaft, with all of those impellers facing in a second direction, opposite the first direction. A fluid passageway interconnects the discharge outlet of the final-stage impeller of the first set with the suction inlet of the first-stage impeller of the second set. The net axial thrust imparted to the rotating shaft is thereby balanced to a relatively low value.
The balanced multi-stage centrifugal pump described above functions generally satisfactorily to accommodate the axial thrusts being generated. However, it is nevertheless subject to drawbacks. A high pressure differential between the final-stage impeller of the first set and the adjacent final-stage impeller of the second set can cause a high-velocity flow of pumped fluid across a breakdown bushing located between these impellers. If the pumpage is at all abrasive, as frequently is the case, this high-velocity flow can lead to erosion of certain pump components and, ultimately, pump failure. Special steps can be taken in the design and construction of the breakdown bushing so as to accommodate this high pressure differential. However, it is not believed that the erosion problem can adequately be solved, and the pump still will have a limited lifetime.
Another technique for accommodating the high axial thrusts provided by multi-stage centrifugal pumps is to balance the thrust hydraulically. This requires the use of a special disk or drum and the accommodating of a high pressure differential by bleeding a high pressure to suction. This bleeding of high pressure pumpage leads to the very same erosion problem discussed above.
It should therefore be appreciated that there is a need for a multi-stage centrifugal pump that can accommodate the high axial thrusts provided by the pump's impellers without the need for a breakdown bushing for sealing a high pressure differential and without the need for undesired hydraulic balancing. The present invention fulfills this need.